An internal floating roof storage tank generally has a solid cylindrical outer wall covered by a solid sloped or dome-shaped roof. An internal floating roof is held within the volume defined by the outer wall and roof. The internal floating roof extends over the liquid contents held within the volume, and forms a vapor seal around the internal circumference of the cylindrical wall. The height within the tank at which the internal floating roof is positioned varies according to the amount of liquid being stored within the tank at any given time.
Internal floating roof storage tanks generally are used to store volatile or flammable liquids, such as gasoline. A vapor space is formed inside the tank, between the internal floating roof covering the gasoline and the outer tank roof. Vapors emitted from the internal volume of the storage tank collect in the volume between the floating roof and the external roof. This volume is vented to the atmosphere. Environmental protection regulations restrict the amount of hydrocarbon vapors that may be released to the atmosphere. Air quality may be impacted adversely if hydrocarbon vapor emissions exceed permitted amounts.
Manual access to the internal volume of the tank is provided within the vapor space. One or more access ports or doorways are formed in the roof or outer wall of the tank, and a ladder or gangway may be supported within the vapor space. Persons may enter the vapor space to service the tank, the internal floating roof and any equipment housed inside the tank. Because of the hazards associated with confined space entry within the vapor space, particularly in flammable atmospheres, entry for inspection purposes is rarely permitted.
UL 913-1988 sets forth the standards that must be satisfied before equipment may be introduced into an internal floating roof storage tank or the vapor space above the floating roof in such tank. “Intrinsically safe” apparatus are protected against ignition by (1) limiting the maximum current and voltage that may be applied so that the energy available in a hazardous location is not capable of igniting the flammable mixture in such location; and (2) taking precautions against spark ignition and thermal ignition that could ignite and create an explosion.
Special precautions are taken when introducing equipment into the internal volume of the internal floating roof storage tank when the tank volume holds a flammable liquid, such as gasoline. Measures are taken to prevent sparks and associated combustion of flammable vapors. Measures are also taken to minimize the amount of time the seal between the flammable liquid and the vapor space above the internal floating roof is broken to prevent substantial amounts of flammable vapors from being emitted into the vapor space.
Internal floating roof storage tanks are inspected at regular intervals to locate cracks, corrosion or other defects that might lead to tank failure. Environmental protection regulations specify the frequency and recommended procedures for inspecting tanks for structural integrity. One method involves introducing a remote controlled submersible vehicle into the tank while the tank remains in service. For example, U.S. Pat. No. 5,205,174 discloses a scavenger submersible vehicle that inspects the internal surfaces of a liquid filled tank using a video camera and/or ultrasound pulses. One or more umbilical hose(s) and wire harness(es) connect the remote-controlled vehicle to its power source and air or vacuum source, and further provide a link for transmitting navigation and inspection data from the vehicle to one or more computers located outside the tank.
Safety and environmental regulations (i.e., 40 CFR 60, subparts K, Ka and Kb) further require that the primary and secondary seals between the periphery of a floating roof and the internal side wall of a tank be inspected at regular intervals. When the tank volume is filled with a hazardous liquid such as a petrochemical like gasoline or other chemical, a cursory visual inspection may be made by a worker who enters the vapor space above the internal floating roof and walks upon the floating roof. Entry into the vapor space poses several dangers, however. First, the vapor space is potentially explosive and extreme care must be taken to prevent heat or sparks that could ignite an explosion. Second, the vapors in the vapor space can be harmful if breathed in.
Remote inspection of the internal floating roof and the primary and secondary seals associated with an internal floating roof storage tank has been accomplished by lowering a camera into the vapor space to photograph regions of the seal. This qualitative method has been used to identify regions that should be visually inspected more closely by a worker as a way to help shorten the time the worker remains in the vapor space.
No method has yet been found to inspect the seal and provide an accurate quantitative assessment of the dimensions of a defect or gap without having a worker enter the vapor space. An accurate remote inspection method continues to be sought.